Software⏱️ 3 min read📅 2026-06-11

How to Fix: Why am I experiencing an HTTP 2.0 protocol error connecting to localhost web sites in IIS?

IIS HTTP/2 protocol error connecting to localhost web sites.

Quick Answer: Try disabling HTTP/2 in IIS settings or updating the .NET Framework and ASP.NET MVC updates.

📋 Table of Contents

  1. ⚠️ Common Causes
  2. 🔧 Proven Troubleshooting Steps
  3. 🎯 Final Words

When connecting to a localhost web site using Visual Studio 2022 Community Edition, you may experience an HTTP 2.0 protocol error due to recent updates on Windows 11 and .NET Framework. This issue affects users who have installed the specified cumulative updates and security patches.

This problem can be frustrating as it prevents you from accessing your localhost web site. However, by following the steps outlined below, you should be able to resolve the issue.

⚠️ Common Causes

  • The primary cause of this error is related to the recent updates on Windows 11 and .NET Framework. Specifically, KB5066835 (26200.6899) introduced changes that may interfere with HTTP/2 protocol functionality.
  • Another possible reason for this issue is the Security Update for Microsoft ASP.NET MVC 5.1 (KB2994397), which also affects HTTP/2 protocol compatibility.

🔧 Proven Troubleshooting Steps

Update Windows 11 and .NET Framework to a previous version

  1. Step 1: Go to Settings > System > About and check the version of Windows 11. If it's 25H2, update to a previous version (e.g., 21H2).
  2. Step 2: Open the Microsoft Update Catalog website ([https://www.microsoft.com/en-us/SoftwareDownload/details.aspx?familyid=3a7f9c95-1b65-4d15-8d66-0e6af5ca33ad](https://www.microsoft.com/en-us/SoftwareDownload/details.aspx?familyid=3a7f9c95-1b65-4d15-8d66-0e6af5ca33ad)) and search for the relevant cumulative update or security patch. Download and install the update.
  3. Step 3: Restart your system after installing the updates.

Disable HTTP/2 protocol in IIS

  1. Step 1: Open the IIS Manager and navigate to the localhost web site configuration.
  2. Step 2: In the 'Actions' panel, click on the 'Edit Application Pool' option for the relevant application pool.
  3. Step 3: In the 'Application Pool' settings, scroll down to the 'HTTP/2 protocol version' section and select 'Off'.
  4. Step 4: Click 'OK' to save the changes.

🎯 Final Words

By following these steps, you should be able to resolve the HTTP 2.0 protocol error connecting to localhost web sites in IIS. If you're still experiencing issues, consider seeking further assistance from Microsoft support or a qualified IT professional.

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